The use of battery powered, high technology appliances is becoming very common, both in the home and in the office, and in vehicles used to commute between the two. Examples include battery powered radar detectors; and other examples may include telecommunications or other devices such as portable dictating equipment, cordless telephones, cellular telephones, pagers, caller identity display devices, laptop computers, calculators, personal entertainment devices, and so on. In each of those instances, operation of the device is, at least to some extent, under the control of a micro-processor which is found within the device, and in which specific programs are embedded. Various functions and operating steps, self-checking operations, or even wake and sleep operations may be under the control of the micro-processor--which, itself, is capable of performing timing operations since it is a clock driven device. However, the continued ability of the device to perform its functions under the control of the micro-processor may, at least in part, depend on the voltage being delivered to the micro-processor and to other circuit elements. Indeed, attempted or continued operation of such a device with insufficient supply voltage could result in unpredictable or erroneous behaviour of a control micro-processor. Moreover, with active telephone devices, there may be a chance of failure of the transmitting portion of the device; or even worse, with the transmitter failing to comply with mandated transmission or energy emission standards and regulations.
In general, most such devices are designed, and the circuit components chosen, to operate at one particular voltage--very often, five volts--notwithstanding that the voltage from the battery which powers the device might vary depending on the nature and state of charge of the battery. Most such devices may draw their operating power from a rechargeable battery or a dry cell or cells, with a predetermined but nominal output voltage. For example, several dry cells or batteries having nominal voltages of from six volts to nine volts, may be used in some devices; while other devices are available which may be powered by only one or two dry cells, having an output voltage in the range of 1.2 to 3.0 volts.
Thus, there will be installed in the micro-processor based device a voltage regulator, whose purpose is to output the specific operating voltage for the device and its circuit components, no matter what the battery voltage may be, within certain limits. For example, a five volt voltage regulator may be installed whose purpose is to provide a five volt output from a nominal nine volt battery--whose terminal voltage might, in fact, range from just over nine volts down to about seven volts, or less, depending on its age and state of charge. However, should the battery voltage from that nine volt battery, for example, fall below some particular value beyond which the voltage regulator cannot be counted on to perform its function properly, then there must be some means whereby the operator of the device becomes aware of that fact so that the battery may be recharged or a new battery installed.
Moreover, and in any event as noted above, the battery voltage that is fed into the input port of the voltage regulator may be higher or lower than the output voltage from the voltage regulator, depending on the battery source and type of regulator. The following discussion is therefore directed to circuits whose operation will proceed and which will monitor the status of the battery voltage, whether that voltage is above or below the regulated voltage, depending on the particular circumstances of any specific circuit or the type of regulator being used.